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Hawaiian Islands Synoptic Discussion and Guidance

958
FXHW60 PHFO 280151
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST TUE JAN 27 2015

.SYNOPSIS...
WEAKENING TRADE WINDS WILL TURN SOUTHEASTERLY ON WEDNESDAY...AS A
WEAK FRONT APPROACHES FROM THE WEST. THIS FRONT WILL BRING SOME
SHOWERS AND LIGHT WINDS TO THE ISLANDS THURSDAY AND FRIDAY...THEN
LIGHT NORTHEASTERLY TRADES WILL FILL BACK ON OVER THE WEEKEND. A
STRONGER FRONT MAY APPROACH THE ISLANDS EARLY NEXT WEEK.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL RIDGE TO THE SOUTH AND EAST
OF THE STATE. A SHORTWAVE TROUGH IS LOCATED NEAR THE DATELINE...
WHICH WILL COME INTO PLAY LATER IN THE WEEK. AT THE SURFACE...THE
RIDGE IS LOCATED A COUPLE HUNDRED MILES NORTH OF KAUAI...WITH A COLD
FRONT 500 MILES TO THE NORTHWEST. 00Z RAOBS FROM HILO AND LIHUE
SHOW INVERSIONS AROUND 5KFT AND PRECIPITABLE WATERS AROUND 0.9
INCHES. EARLY AFTERNOON MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS
SIMILAR VALUES LESS THAN ONE INCH UPSTREAM TO THE NORTHEAST AND
EAST...WITH A NARROW BAND OF SLIGHTLY HIGHER PW VALUES POOLED ALONG
THE FRONT TO THE NORTHWEST. BANDS OF HIGHER MOISTURE ARE ALSO
LOCATED FAR SOUTHEAST OF THE BIG ISLAND.

EVEN THOUGH SURFACE PRESSURES ACROSS THE ISLANDS ARE ONLY DOWN
SLIGHTLY OVER THE PAST 24 HOURS...THE PRESSURE DIFFERENCE BETWEEN
BUOY 51101 AND 51003 HAS DROPPED BY NEARLY 2MB OVER THE SAME PERIOD.
THE SURFACE RIDGE IS WEAKENING AS IT SHIFTS SOUTHWARD OVER THE
STATE...LEADING TO A WEAKENING GRADIENT AND DECREASE IN WIND SPEEDS.
TRADES WILL WEAKEN AND VEER MORE SOUTHEASTERLY BY WEDNESDAY AS THE
RIDGE MOVES SOUTH ACROSS THE STATE. THE GRADIENT RELAXES FIRST
ACROSS THE WESTERN HALF OF THE STATE...AND KAUAI/OAHU MAY SEE RATHER
WIDESPREAD LAND BREEZES BY LATE TONIGHT. THE AIRMASS IS INITIALLY
STABLE WITH A LOW INVERSION. RADAR REFLECTIVITY HAS BEEN MINIMAL BUT
PUBLIC REPORTS AND A COUPLE RAIN GAGES INDICATE THAT THE MOUNTAINS
HAVE BEEN ABLE TO SQUEEZE OUT SOME BRIEF SHOWERS. LOW CLOUDS ARE
PRETTY SOLID ACROSS PORTIONS OF THE STATE THIS AFTERNOON...BUT WE
EXPECT AT LEAST PARTIAL CLEARING AS LAND BREEZES DEVELOP OVERNIGHT.
ONSHORE SEA BREEZES ARE EXPECTED ON WEDNESDAY...LEADING TO AN
INCREASE IN CLOUDS AND LIGHT SHOWERS ACROSS INTERIOR AREAS DURING
THE AFTERNOON.

THE FRONT CURRENTLY NORTHWEST OF THE STATE IS EXPECTED TO WEAKEN AS
IT APPROACHES THE ISLANDS WEDNESDAY NIGHT. THE SHORTWAVE NEAR THE
DATELINE WILL AMPLIFY AND MAY EITHER LEAD TO A SECONDARY FRONT
FORMING BEHIND THE FIRST...AS INDICATED IN THE 12Z/18Z GFS AND
ECMWF...OR JUST A REINTENSIFICATION OF THE ORIGINAL FRONT. EITHER
WAY...THIS FEATURE WILL GRADUALLY PROPEL THE FRONT SOUTHEAST ACROSS
THE ISLANDS THROUGH FRIDAY. THERE IS LIMITED UPPER LEVEL SUPPORT
ASSOCIATE WITH THE FRONT AT THIS LATITUDE...AND MOISTURE WILL BE
MARGINAL. FORECAST SOUNDINGS AND CROSS SECTIONS SHOW INVERSIONS
RAISING/MOISTURE DEPTH INCREASING SLIGHTLY WITH THE FRONT...ALTHOUGH
OVERALL RAINFALL AMOUNTS ARE STILL EXPECTED TO BE RELATIVELY LIGHT.

BOTH THE GFS AND ECMWF SHOW THE SURFACE BOUNDARY STALLING NEAR THE
BIG ISLAND LATE FRIDAY/SATURDAY...WITH SOME ENHANCEMENT FROM THE
SOUTHEAST. WITH THE LOW-LEVEL FLOW TURNING SOUTHEASTERLY AS THE
RIDGE SHIFTS EASTWARD...THE EXISTING CLOUD BANDS ASSOCIATED WITH OLD
FRONTS CURRENTLY SOUTHEAST OF THE BIG ISLAND WILL BE PULLED
NORTHWESTWARD TOWARD THE STATE. THE 12Z MODELS SHOW THESE FEATURES
ENHANCING THE MOISTURE AND LOW-LEVEL VORTICITY ASSOCIATED WITH THE
OLD FRONT...AND LINGERING THIS BAND NEAR THE BIG ISLAND THROUGH THE
WEEKEND. THIS FEATURE IS EXPECTED TO LIFT NORTHWARD EARLY NEXT WEEK
AS THE NEXT SYSTEM APPROACHES FROM THE WEST...BUT IT MAY STILL
ENHANCE MAINLY WINDWARD SHOWERS ACROSS THE BIG ISLAND AND MAUI.

EARLY NEXT WEEK A STRONGER FRONT MAY APPROACH THE ISLANDS...BUT
THERE REMAINS A FAIR AMOUNT OF UNCERTAINTY WITH THIS SYSTEM. THE 12Z
GFS NOW DEVELOPS A LOW MUCH FARTHER SOUTH...TRACKING ALONG
30N...THAT PROPELS A FRONT SOUTHWARD ACROSS THE ISLANDS. EARLIER
RUNS HAD VARYING DEGREES OF FEEDBACK THAT AFFECTED TIMING AND
PLACEMENT AS WELL...BUT STILL FEATURED THE PARENT LOW MUCH FARTHER
NORTH. THE 12Z ECMWF HAS A WEAKER BOUNDARY THAT COMES THROUGH WITH A
SIMILAR TIMING...BUT WITH THE SURFACE WAVE TRACKING CLOSER TO 40N
THAN TO 30N. ONLY ONE GFS ENSEMBLE MEMBER IS CLOSE TO THE STRENGTH
OF THE OPERATIONAL RUN...WITH MOST MUCH WEAKER. THE CURRENT FORECAST
FEATURES WINDS THAT ARE MUCH WEAKER THAN THE OPERATIONAL GFS...WHICH
SEEMS REASONABLE GIVEN ITS OUTLIER STATUS.

&&

.AVIATION...
VFR CONDITIONS WILL PREVAIL ACROSS THE STATE THROUGH WEDNESDAY.
WEAKENING WINDS WILL RESULT IN MORE SEA BREEZE AND LAND BREEZE
OCCURRENCES AS THE CURRENT EAST TO SOUTHEAST WIND FLOW DIMINISHES...
WITH WINDS BECOMING VERY LIGHT WEDNESDAY. THUS EXPECT AFTERNOON
CONVECTIVE CLOUDINESS OVER INTERIOR SECTIONS AND ALONG MOUNTAIN
SLOPES WITH NIGHTTIME CLEARING THROUGH WEDNESDAY NIGHT.

&&

.MARINE...
LONG PERIOD FORERUNNERS OF THE NEXT NORTHWEST SWELL PICKED UP AT THE
WAIMEA BUOY THIS AFTERNOON...WITH SURF REPORTS AND CAMS SHOWING AN
INCREASE IN HEIGHTS AS WELL. BUOY 51101 HAS CONTINUED THE GRADUAL
RISE FROM EARLIER THIS MORNING...AND IS ON PACE WITH THE LATEST
WAVEWATCH III GUIDANCE. BASED ON RECENT REPORTS...WE HAVE DECIDED TO
ISSUE THE HIGH SURF ADVISORY STARTING NOW. GUIDANCE SHOWS THE SWELL
GRADUALLY INCREASING THROUGH LATE WEDNESDAY...THEN DIMINISHING BY
THURSDAY. WITH THIS INCOMING SWELL...COMBINED SEAS MAY ALSO FLIRT
WITH THE 10 FOOT SMALL CRAFT ADVISORY LEVEL FOR SOME AREAS.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY UNTIL 6 AM HST THURSDAY FOR NORTH AND WEST
FACING SHORES OF NIIHAU KAUAI AND OAHU AND NORTH FACING SHORES OF
MOLOKAI AND MAUI.

&&

$$

DISCUSSION/MARINE...BRAVENDER
AVIATION...BF

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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